This invention relates generally to the metal injection molding (MIM) industry, and more specifically, to a composition for use in MIM processes.
MIM processes have generally been expensive in the past due to the relatively high cost of the fine metal powders used in such processes. This cost is about an order of magnitude higher than that of more coarse powders used in conventional press and sinter powder metallurgical processes.
These fine powders have heretofore been considered necessary because the fine particle size provides an extremely high particle surface area. This high surface area is a primary factor in known MIM compositions and processes to allow for densification to a level sufficient to provide satisfactory mechanical and physical properties in the parts produced by MIM processes. Molded and debinderized MIM components typically have about 35-40% porosity and a large amount of shrinkage, or densification, is necessary to reduce the porosity of the finished component to about 1-5% in order to obtain the requisite properties.
The use of more coarse powders, such as those commonly used in powder metallurgy processes, or a mixture of coarse and fine powders, is possible using known MIM compositions. However, the use of coarse powders theoretically requires extending sintering times as much as 10,000 times the time required for fine powders in order to achieve the requisite densification. Therefore, it is desirable, in order to make MIM components more economical to produce, to provide a composition which will allow the use of fine and coarse powders in MIM processes without requiring such extended sintering times.
Known MIM compositions and processes utilize grain boundary or surface diffusion controlled sintering processes. These processes depend on surface energy as the driving force for sintering. Therefore, the fine MIM-type powders exhibit greater shrinkage than the coarser P/M-type powders because of the larger surface area which is characteristic of the fine MIM-type powders. While these surface energy dependent solid state sintering processes work well with the fine powder compositions presently used in MIM processes, new compositions and processes are needed to take advantage of the economic benefits of using coarse powders.